If you believe the headlines and the soundbites, then there is a generation of 25-year-olds who are “failing to launch” into adulthood. Instead of eagerly seizing the obvious badges of maturity – marriage, mortgage, career – these ageing fledglings sit around in their childhood bedrooms, playing video games and refusing to consider any job that pays less than £30,000 a year.

Meanwhile, there is a corresponding flap about the way that their 50-something parents are also refusing to act their age. The men play football after work, the women make cupcakes and together, although they have been married for 30 years, they go on “date nights”. To some commentators this jumbling of developmental markers is unseemly. To others, such as the permanently outraged American conservative commentator Diana West, it is much worse. The title of her 2007 bestseller said it all, and a bit more to boot: The Death of the Grown-up: How America’s Arrested Development is Bringing Down Western Civilisation. This hoo-hah – and that’s all it is,suggests Steven Mintz of the University of Texas – is the result of our foreshortened historical memory. In The Prime of Life Mintz argues that we can’t see back further than the 1950s, a decade during which it was indeed normal to be married, mortgaged and manacled to a career by the time you reached your quarter century. For some odd reason, we assume that how things were in 1955 is how things have always been until about 2005, when everyone suddenly started behaving as if they had been cast in a film starring Adam Sandler.

This sort of wilful amnesia is only possible, Mintz suggests, because we don’t yet have a fully developed history of adulthood, one that lets us recognise the 50s for what they were: a short-lived time during which young people briefly had the means to live like 45-year-olds. In addition, he argues, we’ve held on for far too long to what might be called a Freudian view of the journey to adulthood. According to this model there are certain psychological stages – mostly to do with sublimating aggressive and libidinal impulses – that have to be mastered before maturity is achieved. The problem here is Freud’s ahistoricism. His developmental map may have made sense in 1890s upper-bourgeois Vienna – the good doctor himself was an established career and family man by the time he was 30 – but has little to do with how people lived in colonial America or, for that matter, how they continue to go about life in post‑industrial Britain.

Mintz’s project, then, is to reintroduce the idea of adulthood as a historical artefact, a trajectory shaped by contingent and specific socio-economic circumstances. He does this by providing an elegant survey of recent academic research into cognate matters such as population growth, religious teaching, war and wages, out of which he constructs a detailed account of how people – mostly Americans but Britons too – have historically negotiated the years between 15 and 65.

For instance, in the 19th century it was normal for a young man to live at home until he was in his late 20s, by which time his parents had helpfully died, leaving behind some tools, or cash, or some other capital that would allow him to ask a girl for her hand in marriage. He couldn’t assume, though, that she would say yes. Young women, according to Mintz, have historically been reluctant to wed. Having tasted freedom while at boarding school or serving behind a counter, a sensible girl might wonder whether signing on as someone else’s helpmeet was really such an attractive proposition. Indeed, he goes so far as to say that, in the 19th century, many girls underwent “a marriage trauma”, as they weighed up whether life as an old maid might actually offer the better deal.

Some young people didn’t even get that far. In the 1830s Henry Thoreau exhibited exactly the kind of kidult complex guaranteed to get modern commentators hot under the collar. Following an expensive education at Harvard, Thoreau did some listless tutoring before spending the rest of his working life in his father’s pencil factory. And, although Mintz doesn’t mention this, it’s worth recalling that while, in his late 20s, Thoreau was taking time out on Walden Pond in Concord, Massachusetts, his mother used to pop in every Sunday with a basket of home-cooked meals – a case of helicopter parenting if ever there was one.

But even if Thoreau had managed to achieve some of the outward trappings of adulthood, there is no guarantee that he would have been able to hang on to them for good. Mintz is eloquent on how discontinuities and reversal, rather than stasis or advancement, have been the historical lot of the grown-up. The death of a spouse, the collapse of a bank, chronic ill-health – all those familiar motors of the classic novel, in fact – could entail a dramatic forfeiture of what once seemed like a fully secured maturity.

And that was just the beginning. In the next century the economic collapse of 1929 meant a new generation of jobless men on both sides of the Atlantic barely able to support themselves, let alone a family. It was not until after the second world war, Mintz suggests, with full employment returning, that young men and women could do what they had never been able to do before and haven’t been able to since: marry at 21 and embark on an adulthood defined by enduring social and financial relationships. Anyone who didn’t go down this route was probably delinquent, possibly homosexual and certainly not the sort of person you wanted next to you on the factory floor or in the boardroom.

In the light of this fluxy backstory, Mintz refuses to see anything sinister about today’s situation where the markers of adulthood – learning to drive, losing your virginity, graduating, first job, first child, marriage – are stretched out over a period of 20 years rather than clumped together in one convulsive transformation. He argues that there is much that is beneficial about this extended period of middle youth: less teenage promiscuity than a generation ago (so he claims), better friendships between men and women and, as a consequence, marriages that are based on social and economic equality. What’s more, those who enter the permanent job market later tend to do better in the long term.

However, there is one huge caveat. As Mintz ruefully acknowledges, this extended apprenticeship to adulthood is only available to the reasonably well‑to-do. You need parental support, both emotional and financial, to keep you ticking over until you find that job you love, that flat you can afford, even that man you want to marry. For everyone else the lack of external footholds means that it is increasingly difficult to get any purchase on adult life. In Mintz’s bleak analysis, the poor aren’t simply getting poorer, they are increasingly condemned to a permanent adolescence of the old‑fashioned kind, one that doesn’t involve hunkering down in their parents’ well-appointed games room while they figure out what sort of grown-up they really want to be.

•The Prime of Life by Steven Mintz (Harvard University Press, £25.95). To order a copy for £25.95, go to bookshop.theguardian.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min. p&p of £1.99.